1. Field of the Invention
The present invention relates to a carrier core material for an electrophotographic developer, used in a two-component electrophotographic developer used in apparatuses such as copying machines and printers, a carrier and an electrophotographic developer using the carrier.
2. Description of the Related Art
An electrophotographic development method is a method in which development is conducted by adhering the toner particles in a developer to the electrostatic latent image formed on a photoreceptor, and the developer used in such a method is classified into a two-component developer composed of toner particles and carrier particles and a one-component developer using only toner particles.
As a development method using a two-component developer composed of toner particles and carrier particles, among such developers, previously a method such as a cascade method has been adopted, but currently a magnetic brush method using a magnet roll predominates.
In a two-component developer, the carrier particles serve as a carrying substance to form a toner image on the photoreceptor in such a way that the carrier particles are stirred together with the toner particles in a developer box filled with the developer to impart a desired charge to the toner particles, and further, convey the thus charged toner particles to the surface of the photoreceptor to form the toner image on the photoreceptor. The carrier particles remaining on a development roll which holds a magnet again return from the development roll to the developer box to be mixed and stirred with the fresh toner particles to be repeatedly used for a predetermined period of time.
In contrast to a one-component developer, a two-component developer is such that the carrier particles are mixed and stirred with the toner particles, thus charge the toner particles, and further have a function to convey the toner particles, and a two-component developer is excellent in the controllability in designing developers. Accordingly, two-component developers are suitable for full-color development apparatuses required to offer high image quality and for high speed printing apparatuses required to be satisfactory in the reliability and durability in image maintenance.
In two-component developers used in the above-described manner, the image properties such as the image density, fogging, white spot, gradation and resolution are each required to exhibit a predetermined value from the initial stage, further these properties are required to be invariant and to be stably maintained during the endurance printing. For the purpose of stably maintaining these properties, necessary is the stability of the properties of the carrier particles contained in the two-component developers.
As the carrier particles which form two-component developers, there have hitherto been used iron powder carriers such as an iron powder carrier in which the surface of an iron powder is coated with an oxide coating film or a resin. Such iron powder carriers are high in magnetization and also high in conductivity, and hence have an advantage that images satisfactory in the reproducibility of the solid print portions thereof are easily obtained.
However, the true specific gravities of such iron powder carriers are as heavy as approximately 7.8, and the magnetizations of such iron power carriers are too high. Accordingly, the stirring and mixing of such an iron powder carrier with the toner particles in the developer box tend to cause the fusion bonding of the toner-constituting components to the surface of the iron powder carrier, namely, the so-called toner spent condition. The occurrence of such a toner spent condition reduces the effective surface area of the carrier, and the triboelectric charging ability of the carrier in relation to the toner particles tends to be degraded.
Additionally, in the resin-coated iron powder carrier, the resin on the surface is exfoliated by the stress at the time of endurance to expose the core material (iron powder) which is highly conductive and low in dielectric breakdown voltage, and accordingly the charge leakage occurs as the case may be. Such charge leakage breaks the electrostatic latent image formed on the photoreceptor, causes brush strokes or the like to occur on the solid print portion, and makes it difficult to obtain a uniform image. Due to these reasons, currently the iron powder carriers such as oxide film-coated iron powder carriers and resin-coated iron powder carriers have become decreasingly used.
In these years, in place of the iron powder carriers, ferrites each having a true specific gravity of as light as approximately 5.0 and being low in magnetization are used as carriers, and further, resin-coated ferrite carriers in each of which the surface of the ferrite is coated with a resin are frequently used, and accordingly the operating lives of the developers have been dramatically extended.
As methods for producing such ferrite carriers, in general, a ferrite carrier raw material is mixed in a predetermined amount, thereafter calcined, pulverized and granulated, and thereafter sintered; the calcination may be omitted depending on the conditions involved.
However, such methods for producing ferrite carriers suffer from various problems. Specifically, the sintering step of generating magnetization by ferritization reaction generally uses a tunnel kiln to sintere a raw material filled in a saggar, and hence, due to the inter-particle effects, the ferrite particles tend to be irregular in shape; in particular, the irregularity is the more remarkable for the smaller particles; thus, after sintering, particles are block shaped, and when the blocks are disintegrated, cracking and chipping occur to result in commingling of irregular particles. Additionally, when a small-size ferrite particle is produced, enhanced pulverization is essential for production of satisfactorily shaped particles. Furthermore, problematically, the sintering time is required to be approximately 12 hours inclusive of a period for increasing the temperature, a period for maintaining the maximum temperature and a period for decreasing the temperature; and in addition, after sintering, disintegration of the block-shaped material is needed, and thus the production stability is not satisfactory.
Additionally, the carrier core materials produced by means of such a sintering method as described above include abundantly irregular particles due to particle deformation in addition to cracked and chipped particles, and hence the formation of a resin coating film also finds it difficult to form a uniform coating film. The resin coating film becomes thicker in the recessed portions on the particle surface and becomes thinner in the raised portions on the particle surface. The portions where the thickness of the resin coating film is thinner are earlier, due to the stress, in exposing the carrier core material and becomes the causes for the leakage phenomenon and the broadening of the charge amount distribution, and thus, it has been difficult to stabilize high image qualities over a long period of time.
For the purpose of preventing the cracking and chipping, and reducing the proportion of the irregular particles, it is necessary to prevent the mutual aggregation of the particles at the time of sintering. When the particles are sintered at a lower sintering temperature for the purpose of preventing such aggregation, the stress at the time of the disintegration after sintering becomes smaller and the proportion of the cracked and chipped particles and the proportion of the irregular particles and the like can be reduced.
However, in this case, the surface property of the particles becomes porous, the resin impregnation or the like degrades the charging rise, and the amount of the resin in the impregnated portions, namely, the amount of the resin unnecessarily consumed becomes larger, and this case is poor in economic efficiency and is not desirable from both quality and cost.
For the purpose of solving such problems, a new method for producing a ferrite carrier has been proposed. For example, Japanese Patent Laid-Open No. 62-50839 describes a method for producing a ferrite carrier in which a mixture composed of metal oxides mixed therein as the raw materials for the ferrite is made pass through a high temperature flame atmosphere, and thus the mixture is instantly ferritized.
However, in this production method, the production is conducted with a ratio of the oxygen amount to the fuel gas of 3 or less, and accordingly, the sintering becomes difficult depending on the ferrite raw materials. Additionally, in the production of a ferrite having a small particle size of, for example, approximately 20 to 50 μm, to meet the recent trend of particle size reduction of carriers, the heat amounts applied to the individual particles are possibly different from each other, no sufficient heat is possibly applied to the interior of the relatively larger particles having a particle size of 50 μm or more, and thus, in either case, no spherical and uniform ferrite particles are obtained.
Additionally, International Publication No. 2007-63933 describes a method for producing a resin-coated ferrite carrier in which a thermal spray method as described above is used, a combustion gas and oxygen are used for the flammable gas combustion flame, and the volume ratio of the combustion gas to oxygen is set at 1:3.5 to 1:6.0; the resin-coated ferrite carrier thus produced is stated such that the surface of the carrier core material is provided with irregularities which form a thin stripe-like wrinkle pattern to improve the adhesion strength of the surface of the carrier core material to the resin coating film.
This International Publication No. 2007-63933 states that various ferrite compositions can be used as the ferrite compositions used as the carrier core materials; however, as is clear from the examples presented therein, the ferrite composition disclosed as the carrier core material is limited to a Mn—Mg—Sr ferrite.
On the other hand, in these years, the environmental regulations become strict, the use of metals such as Ni, Cu and Zn has come to be avoided, and the use of the metals adaptable to the environmental regulations is demanded. Thus, the ferrite compositions used as the carrier core materials are changing over from the Cu—Zn ferrite and the Ni—Zn ferrite to the ferrites using Mn such as the Mn ferrite and the Mn—Mg—Sr ferrite.
However, Mn is also becoming the objects of various legal regulations, and carrier core materials which do not use Mn as well as the above-described various heavy metals are demanded.
On the other hand, as the ferrite composition free from the use of Mn, various proposals have been made on the compositions such as a Li—Mg—Ca ferrite and a Mg ferrite, but these compositions are still unfamiliar. As a reason for this, for example, it is known that the ferrite carrier using Li as a carrier core material undergoes large variations of the charge amount and the resistivity due to the effect of a slight amount of moisture in the air even when the surface of the carrier is coated with a resin. Additionally, the Mg ferrite requires, as a prerequisite, a sintering under a controlled atmosphere, depending on the addition amount of Mg, for the purpose of attaining satisfactory properties; thus, it is extremely difficult to produce the Mg ferrite when only equipment for sintering in the atmosphere is available.
Japanese Patent Laid-Open Nos. 2004-279883, 2004-191834 and 2004-53643 propose electrophotographic developer carriers including ferrite compositions using rare earth metals such as Dy, Tb and Gd, respectively, without using Mn.
These carriers do not use Mn, but each use a rare earth metal; such rare earth metals are hardly available and high in price, and hence are difficult to use for industrial applications.
Japanese Patent Laid-Open No. 2000-233930 describes the inclusion of Ti for the purpose of stabilizing the spinel phase, but the addition of Ti is not for the purpose of positively controlling the magnetization.
Japanese Patent Laid-Open No. 2004-240321 describes the incorporation of an element such as Ti so as to be present in a ferrite as a single oxide, but this incorporation is not for the purpose of controlling the magnetization by producing a composite oxide including Fe and Ti, other than the spinel.
As described above, demanded are: a carrier core material which, without using Mn as well as various heavy metals, attains desired properties, in particular, attains a desired resistivity in combination with a low magnetization, has a coercive force of the order of magnitude not to affect the fluidity, and has a satisfactory fluidity; a carrier; and an electrophotographic developer using the carrier.